The present invention relates to an image sensor; and, more particularly, to an image sensor and a method for driving the image sensor, which is capable of reducing power consumption and improving a picture quality.
Generally, an image sensor is an apparatus to capture images using light sensing semiconductor materials. The image sensor includes a pixel array which contains a plurality of image sensing elements, e.g., photodiode and receives light from an object to generate an electric image signal.
Such an image sensor is disclosed in a copending commonly owned application, U.S. Ser. No. 09/305,756, entitled xe2x80x9cCMOS IMAGE SENSOR HAVING COMMON OUTPUTTING TRANSISTORS AND METHOD FOR DRIVING THE SAMExe2x80x9d filed on May 10, 1999, and has a control and system interface unit.
The control and system interface unit controls the image sensor by controlling an integration time, scan addresses, operation modes, a frame rate, a bank and a clock division, and serves as an interface with an external system.
A pixel array including MxN unit pixels, arranged in a matrix, senses images from an object. The image sensor generally employs a correlated double sampling (hereinafter, referred to as a CDS), to thereby obtain high picture quality under the control of the control and system interface. In order to implement the CDS, each of the unit pixels includes, e.g., a photodiode and four transistors, respectively. Also, the four transistors in the unit pixel include a transfer transistor, a reset transistor, a drive transistor and a select transistor. According to the CDS, the unit pixel outputs a reset voltage level as a unit pixel output signal from a voltage source by turning on the select transistor while the reset transistor is kept on a turned-on state under the control of the control and system interface unit. Also, the unit pixel provides a data voltage level as another unit pixel output signal from the photodiode by turning on and off the transfer transistor in a turned-off state of the reset transistor and reading out the photoelectric charges generated in the photodiode under the control of the control and system interface unit. As a result, an unexpected voltage in the unit pixel can be effectively removed and a net image data value can be obtained by using the reset voltage level and the data voltage level as unit pixel output signals.
Referring to FIG. 3, there is shown a timing chart showing the control scheme for the CDS.
The turned-on transfer transistor is turned off and kept on a turned-off state during a predetermined period in response to the transfer control signal Tx1, while the reset transistor and the select transistor are kept on the turned-on state and the turned-off state, respectively. As shown, the select transistor is turned on in response to a select control signal Sx1 while the reset transistor is kept on the turned-on state and the transfer transistor is kept on the turned-off state, so that a reset voltage level is outputted through the drive transistor and the select transistor as a unit pixel output signal.
However, in the this case, the reset transistor and the select transistor should be simultaneously turned on in order to output the reset voltage level through the drive transistor and the select transistor. At this time, a current path between the voltage source and a ground can be formed, so that a leakage current is unnecessarily flowed. Therefore, undesirable power consumption may be incurred by the leakage current.
Furthermore, as shown in FIG. 4, the power line 310 is arranged around the pixel array 300. The leakage current caused in the section xe2x80x9cB2xe2x80x9d may not affect unit pixels adjacent to the power line 310. However, in case of unit pixels disposed in the middle of the pixel array, i.e., a unit pixel 320, since the power consumption caused by the leakage current occurs, the lowered voltage level may be supplied to the unit pixel 320. Therefore, there may be a undesirable problem that a level of the image value to be outputted also is lowered to thereby degrade the total picture quality in the image sensor.
It is, therefore, an object of the present invention to provide a method for driving a pixel array, which is capable of improving a picture quality by reducing the unnecessary power consumption.
It is, therefore, another object of the present invention to provide an image sensor, which is capable of improving a picture quality by reducing the unnecessary power consumption.
In accordance with an aspect of the present invention, there is provided a method for driving a pixel array including a plurality of unit pixels in an image sensor, wherein each of the unit pixels has a light sensing means for receiving light from an object and for generating photoelectric charges, an amplification means for amplifying an input signal to output an amplified signal, a first switching means for transferring the photoelectric charges from the light sensing means as the input signal to the amplifying means, a second switching means for transferring a reset voltage level from a voltage source to the light sensing means and the amplification means, and a third switching means for outputting the amplified signal and the reset voltage level as unit pixel output signals, the method comprising the steps: a) controlling the first switching means, the second switching means and the third switching means to provide the reset voltage level to the light sensing means in order to make a fully depleted region in the light sensing means; b) after a first predetermined period from the completion of the step a), turning off the second switching means, while the first switching means and the third switching means are kept on the turned-off state; and c) within a second predetermined period from the completion of the step b), turning on the third switching means, while the first switching means and the second switching means are kept on the turned-off state, to thereby output the reset voltage level as a unit pixel output signal.
In accordance with another aspect of the present invention, there is provided an image sensor for converting an image into an electrical signal, comprising: a pixel array including a plurality of unit pixels, arranged in a matrix, wherein each of the unit pixels for generating an amplified signal corresponding to photoelectric charges and a reset voltage level as unit pixel output signals, each of the unit pixels including: a light sensing means for receiving light from an object and for generating photoelectric charges; an amplification means for amplifying an input signal to output the amplified signal and the reset voltage signal; a first switching means, in response to a first control signal, for transferring the photoelectric charges from the light sensing means as the input signal to the amplification means; a second switching means, in response to a second control signal, for transferring the reset voltage level from the voltage source to the light sensing means and the amplification means; and a third switching means, in response to a third control signal, for outputting the amplified signal and the reset voltage level as unit pixel output signals; and a control means for controlling the pixel array by generating the first, the second and the third control signals in accordance with a control mode, wherein the control mode includes a first unit pixel output control mode and, in the first unit pixel output control mode, the second switching means is turned off in response to the second control signal and the third switching means is turned on in response to the third control signal within a predetermined period after turning-off of the second switching means, while the first switching means is kept on a turned-off state, to thereby output the reset voltage level through the amplification means and the third switching means as an unit pixel output signal.